Field of the Invention
The present invention relates to implantable medical devices, and more particularly to stents that are implanted within a bodily lumen of a living animal or human to support the lumen and maintain patency, and that have openings containing a drug for delivery to the body.
Summary of the Related Art
In the past, permanent or biodegradable devices have been developed for implantation within a body passageway to maintain patency of the passageway. These devices are typically introduced percutaneously, and transported transluminally until positioned at a desired location. These devices are then expanded either mechanically, such as by the expansion of a mandrel or balloon positioned inside the device, or expand themselves by releasing stored energy upon actuation within the body. Once expanded within the lumen, these devices, called stents, become encapsulated within the body tissue and remain a permanent implant.
Known stent designs include monofilament wire coil stents (U.S. Pat. No. 4,969,458); welded metal cages (U.S. Pat. Nos. 4,733,665 and 4,776,337); and, most prominently, thin-walled metal cylinders with axial slots formed around the circumference (U.S. Pat. Nos. 4,733,665; 4,739,762; and 4,776,337). Known construction materials for use in stents include polymers, organic fabrics and biocompatible metals, such as, stainless steel, gold, silver, tantalum, titanium, and shape memory alloys, such as Nitinol.
U.S. Pat. No. 6,241,762 which is incorporated herein by reference in its entirety discloses a non-prismatic stent design which remedies several performance deficiencies of previous stents. In addition, preferred embodiments of this patent provide a stent with large, non-deforming strut and link elements, which can contain holes without compromising the mechanical properties of the strut or link elements, or the device as a whole. Further, these holes may serve as large, protected reservoirs for delivering various beneficial agents to the device implantation site without the need for a surface coating on the stent.
Of the many problems that may be addressed through stent-based local delivery of beneficial agents, one of the most important is restenosis. Restenosis is a major complication that can arise following vascular interventions such as angioplasty and the implantation of stents. Simply defined, restenosis is a wound healing process that reduces the vessel lumen diameter by extracellular matrix deposition and vascular smooth muscle cell proliferation and which may ultimately result in renarrowing or even reocclusion of the lumen. Despite the introduction of improved surgical techniques, devices and pharmaceutical agents, the overall restenosis rate for bare metal stents is still reported in the range of 25% to 50% within six to twelve months after an angioplasty procedure. To treat this condition, additional revascularization procedures are frequently required, thereby increasing trauma and risk to the patient.
Conventional stents with surface coatings of various beneficial agents have shown promising results in reducing restenosis. U.S. Pat. No. 5,716,981, for example, discloses a stent that is surface-coated with a composition comprising a polymer carrier and paclitaxel (a well-known compound that is commonly used in the treatment of cancerous tumors). The patent offers detailed descriptions of methods for coating stent surfaces, such as spraying and dipping, as well as the desired character of the coating itself: it should “coat the stent smoothly and evenly” and “provide a uniform, predictable, prolonged release of the anti-angiogenic factor.” Surface coatings, however, can provide little actual control over the release kinetics of beneficial agents. These coatings are necessarily very thin, typically 5 to 8 microns deep. The surface area of the stent, by comparison is very large, so that the entire volume of the beneficial agent has a very short diffusion path to discharge into the surrounding tissue. The resulting cumulative drug release profile is characterized by a large initial burst, followed by a rapid approach to an asymptote, rather than the desired “uniform, prolonged release,” or linear release.
Increasing the thickness of the surface coating has the beneficial effects of improving drug release kinetics including the ability to better control drug release and to allow increased drug loading. However, the increased coating thickness results in increased overall thickness of the stent wall. This is undesirable for a number of reasons, including difficulty in placing the stent in tight lesions, increased trauma to the vessel lumen during implantation, reduced flow cross-section of the lumen after implantation, and increased vulnerability of the coating to mechanical failure or damage during expansion and implantation. Coating thickness is one of several factors that affect the release kinetics of the beneficial agent, and limitations on thickness thereby limit the range of release rates, durations, and the like that can be achieved.
The filled hole or reservoir technology described in U.S. Publication No. 2003/0082680, filed Sep. 7, 2001, and U.S. Publication No. 2004/0073294, filed May 28, 2003, provide a solution to the problem of controlling release kinetics from a stent. The reservoirs allow the deposition of multiple deposits of different polymer only and drug/polymer to achieve a wide variety of release kinetics which cannot be achieved from a surface coating.
Human clinical trials of drug eluting stents have shown that not only is the dose important to the clinical outcome, but the release rate and the increase or decrease in the release rate (the release kinetics) is important in to the clinical outcome. When the release kinetic can be specifically tailored to the biological processes addressed by the stent (i.e. restenosis, inflammation, acute myocardial infarction) the clinical outcome is expected to be improved.